Volcanic deformation at Unzen, Japan, visualized by a time-differential stereoscopy

Abstract Using a pair of old and new photographs taken at the same location, a method named as a time-differential stereoscopy is successfully applied to detect visually the deformation at Unzen volcano, Japan. On September 6, 1992, gradual movement of a lava lobe was visible in a stereo image as relief surrounded by immobile areas. In this case, accelerated movement of a lava block was detected immediately before the time of its collapse, which resulted in the occurrence of a pyroclastic flow. In November 1994, growth of a lava spine was also measured with time-differential stereoscopy. A possible fluctuation of the growth rate might have correlated with an abnormal oscillation of tilt, with a period of about 40 h. This convenient photographic method is recommended for various occasions to detect volcanic and any other deformations as a supplement to precise instrumental measurements.